|Publication number||US7346251 B2|
|Application number||US 11/405,732|
|Publication date||Mar 18, 2008|
|Filing date||Apr 18, 2006|
|Priority date||Apr 18, 2005|
|Also published as||US20070025673, US20080315177|
|Publication number||11405732, 405732, US 7346251 B2, US 7346251B2, US-B2-7346251, US7346251 B2, US7346251B2|
|Inventors||Ranojoy Bose, Chee Wei Wong, Xiaodong Yang|
|Original Assignee||The Trustees Of Columbia University In The City Of New York|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (5), Non-Patent Citations (1), Referenced by (37), Classifications (9), Legal Events (5)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 60/672,398, filed on Apr. 18, 2005, entitled “White light emission using quantum dot emitters in a photonic crystal slab,” which is hereby incorporated by reference herein in its entirety.
1. Field of the Invention
The present invention relates to photonic crystals, and more specifically to producing white light from photonic crystals.
2. Description of the Related Art
Photonic crystals (PC's) are materials, such as semiconductors, that prohibit the propagation of light within a frequency bandgap through an artificial periodicity in their refractive index. In two dimensional photonic crystals, a common way to achieve this artificial periodicity is to periodically arrange air holes (i.e., to have a lattice of air holes) in the material making up the photonic crystal (i.e., silicon or GaN).
Photonic crystals may also contain defects (or cavities). These are locations within a lattice of air holes where one or more air holes are not present. These defects may be created using photolithography techniques. Photonic crystals can be seen as an optical analogue of electronic crystals that exhibit bandgaps due to periodically changing electronic potentials. By introducing a defect within a PC, one or more highly localized electromagnetic modes may be supported within the bandgap (analogous to impurity states in solid state devices). These defects greatly modify the spontaneous emission of light from photonic crystals.
Embodiments of the present invention describe a method of manufacturing a device, and a method of producing substantially white light using a photonic crystal. The photonic crystal has a lattice of air holes and is made from a substrate containing quantum dots. The substrate contains 3 defects that are optically coupled together so that each defect produces only one (or a narrow bandwidth) frequency of light. In combination, these coupled defects can produce substantially white light when the photonic crystal is either optically of electrically pumped.
The photonic crystal is dimensioned so as to cause the optical coupling between the defects to produce substantially white light. These parameters may be determined by using a numerical computation software program such as MIT Photonic Bands (MPB). MPB can be used to compute the resonance modes of coupled defects when given parameters of a photonic crystal.
Embodiments of the present invention contemplate is a method for manufacturing a photonic crystal configured to generate white light. Its substrate is manufactured to include quantum dots, and on this substrate there is fabricated a layer of photonic crystal formed by a lattice of air holes. The lattice of air holes has three defects that are optically coupled together, and dimensioned (in combination with the other dimensions of the photonic crystal) to cause the photonic crystal to produce substantially white light. The defects may be the same size. The parameters of the photonic crystal may be recalculated, if necessary, to cause the photonic crystal to produce substantially white light. The device may also have a cladding layer. One example choice of materials for the photonic crystal is GaN (gallium nitride) for the substrate and InGaN (indium gallium nitride) for the quantum dots. The device may also have a waveguide for guiding the path of the emitted light to an external device.
In various embodiments of the present invention, the photonic crystal can be used to mix a first color of light with a second color of light to produce a third color of light. This example embodiment has only two optically coupled defects instead of three. The defects are disposed and dimensioned to produce two different wavelengths of light that mix together to produce a third wavelength of light. This device may also have a cladding layer and a waveguide.
Various objects, features, and advantages of the present invention can be more fully appreciated with reference to the following detailed description of the invention when considered in connection with the following drawings, in which like reference numerals identify like elements:
Each embodiment of the present invention described below addresses methods for producing substantially white light using a photonic crystal. Embodiments of the present invention can either operate as a stand alone device for producing white light, or as part of a system that utilizes the output of a stand alone device to achieve other goals.
Defects (cavities) that are located close to each other can become optically coupled as known in the art. The light confined in each defect can interact with other defects through the evanescent part of their wavefunctions. This is analogous to carrier tunneling in semiconductor devices. Accompanied with this coupling is a splitting of the resonant mode into three modes with unique frequencies. The extent of this coupling determines the splitting. The coupling between defects may be used to mix different frequencies of light.
Coupling between defects is based on many factors, including as described above, how close the defects are located to each other. Therefore, the interaction of light between coupled cavities may be affected by changing various parameters of the photonic crystal. For example, parameters of the photonic crystal that may be changed include the size and location of defects, the size and location of air holes, the spacing of the air holes, the thickness of the photonic crystal, the material of the photonic crystal, the material and size of the quantum dots, and others as known in the art.
The effect of changing these parameters may vary. For example, changing the size of air holes located near the defects may change the cavity resonances and shift the bandgaps of those defects. As another example, tuning the defect size changes the frequencies of their cavity modes. Yet another example is that changing the radius of one or more of the air holes between the coupled cavities may change the mode-splitting.
Additionally, although only one set of three defects has been shown, a photonic crystal could have an entire array of defect groups (if sufficiently separated to reduce coupling between the groups). Each of these defect groups could then produce white light. Collecting the produced light from these cavities may need to be done individually. This may be done, for example, by using a filter for each group. The filter allows emissions to be collected from only one defect group. An array of defect groups can also be created use multiple substrates, with each substrate containing one or more defect groups.
Computation can be performed using MPB or other similar tools for a GaN PC, which has a refractive index of approximately 2.4 for visible light. MPB can be used to determine the parameters of the photonic crystal that would produce photon emissions corresponding to a set or desired wavelengths. One set of desired wavelengths is 603 nm, 580 nm, and 559 nm, which correspond to normalized frequencies of the cavity field modes of 0.387039, 0.403035 and 0.418253. Normalized frequencies can be computed from a wavelength by the formula f=a/ÿ, where f is the normalized frequency, a is the lattice parameter, and ÿ, is the wavelength. The lattice parameter a defines the center to center spacing between air holes. MPB can be used to compute normalized frequencies of the cavity field modes based on the parameters of a photonic crystal. When the computed normalized frequencies correspond to the desired ones, the photonic crystal parameters have been determined. For example, parameters that may be provided to the MPB program include, the lattice parameter a, the diameter of the air holes, the thickness of the photonic crystal, and the refractive index of the substrate. Other parameters may be determined from a using knowledge commonly known in the art. For example, given a, one estimate for the diameter of the air holes is 0.58a. Similarly, one estimate for the thickness of the photonic crystal is 0.6a. The refractive index of the substrate may be determined from reference sources known to those of skill in the art.
For example, MPB or other similar tools can be used to determine the resonance modes of a set of defects, using for example the value of 234 nm for a lattice parameter a, 133.4 nm (0.58 α) for the diameter of the air holes, 138 nm (0.6 α) for the thickness of the photonic crystal, and ˜3.4 for the refractive index (for a GaN substrate). Example computed frequencies can be 0.382645 (f1), 0.391642 (f2), and 0.406326 (f3), which are close to the desired normalized frequencies of the cavity field modes of 0.387039, 0.403035 and 0.418253. Tuning of the photonic crystal parameters can be used to adjust for any deviation between the desired normalized frequencies, and the one obtained from simulation. The above parameters result in three substantially identically sized defects, due to the even spacing of the lattice. However, even though the defects are the same size, because of the coupling between them (which is a function of the photonic crystal parameters) they each have different bandgaps resulting in the emission of different wavelengths.
As shown in
The interaction between cavity fields with matter placed within them is generally explained through the Purcell Effect. For quantum dots placed within PC cavities, a modification of spontaneous emission occurs, with resonant transitions enhanced and off-resonance transitions suppressed. In weak coupling, the spontaneous emission is enhanced by the Purcell enhancement factor:
F=3Qλ 3ε0/(4πVε m)
where Em represents the effective dielectric constant, V represents the effective modal volume, and ε0 is the permittivity of free space. Depending on the confinement of the cavity—given by a quality factor Q—a resonant photon may be emitted and reabsorbed by the quantum dot, resulting in Rabi oscillations in the strong-coupling limit. The quality factor Q is a measure of the lifetime of the energy within the cavity. It may represent the number of periods before the energy within the cavity decays by a certain factor, for example, by e−2n. The structure may be modified to increase the Q by changing the defect for enhanced emission. Photonic crystal layers may have quality factors as high as 600,000.
Quantum dots (QDs) are semiconductor nanoparticles that confine charge carriers in 3-dimensions. The electrons and holes reside in highly quantized energy states, and hence the resonant transitions (energy to excite a hole in the valence band across the bandgap to the conduction band) are extremely well defined. To emit light in the visible wavelength, one choice of material for the quantum dots is InGaN, which has its energy bandgap in the visible domain. For an ensemble of InGaN quantum dots grown on a GaN substrate, the quantum dots emit photons at very close optical wavelengths (approximately 480 nm to approximately 650 nm).
When an ensemble of quantum dots is placed within a defect, only certain wavelengths of light will be emitted. These emitted wavelengths depend on the parameters of the photonic crystal. As described above, these parameters can be set so that only the desired wavelengths of light are emitted from the defects. In
One way of producing a device containing quantum dots within defects is to etch the defects into a substrate already containing quantum dots. This method relies on self-assembly of quantum dots. Using this self-assembly method, a PC can be etched into a GaN substrate containing InGaN quantum dots. This substrate may also be capped with a GaN cladding layer as taught by K. Hennessy et. al., Proc. SPIE, (2004), which is hereby incorporated by reference herein in its entirety. Hennessy et. al. describes capping a cavity to better confine an optical field in order to maximize the strength of coupling between the cavity and a quantum dot. To create the defects, photolithography can be used to etch the lattice of air holes into the substrate. The defects are then created by not etching one or more holes in certain locations. The air holes that surround the locations then create resonance cavities. Alternatively, quantum dots can be deposited on a substrate using a colloidal containing quantum dots. This embodiment can be used to cover the substrate by, for example, spin coating.
The produced light from the defects would generally be emanating from the same surface of the photonic crystal as from which the photonic crystal was pumped. However,
Alternatively, the defects may be electrically pumped. For example, Park et. al., Characteristics of Electrically Driven Two-Dimensional Photonic Crystal Lasers, IEEE Journal of Quantum Electronics, Vol. 41, No. 9, September 2005, which is hereby incorporated by reference herein in its entirety, describes one method of electrically pumping a photonic crystal cavity. Park et. al. described using a central post placed under a cavity as a first contact, and a metal contact around the cavity as a second contact. Electrically pumping the photonic crystal can result in very similar emissions to those produced by optically pumping. Other methods, that are well known in the art, are also referred to in Park et. al. For example, Zhou et. al., Characteristics of a Photonic Bandgap Single Defect Microcavity Electroluminescent Device, IEEE Journal of Quantum Electronics, Vol. 37, No. 9, September 2001, which is hereby incorporated by reference herein in its entirety, describes etching p and n contacts using optical lithography.
Other embodiments, extensions, and modifications of the ideas presented above are comprehended and within the reach of one versed in the art upon reviewing the present disclosure. Accordingly, the scope of the present invention in its various aspects should not be limited by the examples and embodiments presented above. The individual aspects of the present invention, and the entirety of the invention should be regarded so as to allow for such design modifications and future developments within the scope of the present disclosure.
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|U.S. Classification||385/129, 359/332|
|Cooperative Classification||B82Y20/00, G02B6/1225, B82Y10/00|
|European Classification||B82Y10/00, B82Y20/00, G02B6/122P|
|Oct 17, 2006||AS||Assignment|
Owner name: TRUSTEES OF COLUMBIA UNIVERSITY IN THE CITY OF NEW
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:BOSE, RANOJOY;WONG, CHEE WEI;YANG, XIAODONG;REEL/FRAME:018405/0066
Effective date: 20061009
|Aug 18, 2011||FPAY||Fee payment|
Year of fee payment: 4
|Oct 30, 2015||REMI||Maintenance fee reminder mailed|
|Mar 18, 2016||LAPS||Lapse for failure to pay maintenance fees|
|May 10, 2016||FP||Expired due to failure to pay maintenance fee|
Effective date: 20160318